In vitro binding of the bacteriophage f1 gene V protein to the gene II RNA-operator and its DNA analog.

We have investigated the binding of the f1 single-stranded DNA-binding protein (gene V protein) to DNA oligonucleotides and RNA synthesized in vitro. The first 16 nucleotides of the f1 gene II mRNA leader sequence were previously identified as the gene II RNA-operator; the target to which the gene V protein binds to repress gene II translation. Using a gel retardation assay, we find that the preferential binding of gene V protein to an RNA carrying the gene II RNA-operator sequence is affected by mutations which abolish gene II translational repression in vivo. In vitro, gene V protein also binds preferentially to a DNA oligonucleotide whose sequence is the DNA analog of the wild-type gene II RNA-operator. Therefore, the gene V protein recognizes the gene II mRNA operator sequence when present in either an RNA or DNA context.     

Design, biochemical, biophysical and biological properties of cooperative antisense oligonucleotides.

Short oligonucleotides that can bind to adjacent sites on target mRNA sequences are designed and evaluated for their binding affinity and biological activity. Sequence-specific binding of short tandem oligonucleotides is compared with a full-length single oligonucleotide (21mer) that binds to the same target sequence. Two short oligonucleotides that bind without a base separation between their binding sites on the target bind cooperatively, while oligonucleotides that have a one or two base separation between the binding oligonucleotides do not. The binding affinity of the tandem oligonucleotides is improved by extending the ends of the two oligonucleotides with complementary sequences. These extended sequences form a duplex stem when both oligonucleotides bind to the target, resulting in a stable ternary complex. RNase H studies reveal that the cooperative oligonucleotides bind to the target RNA with sequence specificity. A short oligonucleotide (9mer) with one or two mismatches does not bind at the intended site, while longer oligonucleotides (21mers) with one or two mismatches still bind to the same site, as does a perfectly matched 21mer, and evoke RNase H activity. HIV-1 inhibition studies reveal an increase in activity of the cooperative oligonucleotide combinations as the length of the dimerization domain increases.     

Chemical modification of pyrimidine TFOs: effect on i-motif and triple helix formation

In order to form more stable triple helical structures or to prevent their degradation in cells, oligonucleotide analogs are routinely used, either in the backbone or among the bases. The target sequence chosen for this study is a 16-base-long oligopurine-oligopyrimidine region present in the human neurotrophin 4/5 gene. Seven different chemical modifications were tested for their effect on (i) triple helix formation and (ii) i-DNA stability. i-DNA is a tetrameric structure involving hemiprotonated C x C+ base pairs, which may act as a competing structure for triplex formation, especially in the case of a cytosine-rich third strand. At acid pH, oligophosphoramidates formed the most stable triple helix, whereas oligonucleotides including 5-propynyl-dU formed a stable i-motif which precluded triplex formation. Only two candidates stabilized triple helices at neutral pH: oligonucleotides with phosphoramidate linkage and phosphodiester oligonucleotides containing 5-methyl-dC and 5-propynyl-dU.     

Thermodynamic and Kinetic Characterization of Antisense Oligodeoxynucleotide Binding to a Structured mRNA

Antisense oligonucleotides act as exogenous inhibitors of gene expression by binding to a complementary sequence on the target mRNA, preventing translation into protein. Antisense technology is being applied successfully as a research tool and as a molecular therapeutic. However, a quantitative understanding of binding energetics between short oligonucleotides and longer mRNA targets is lacking, and selecting a high-affinity antisense oligonucleotide sequence from the many possibilities complementary to a particular RNA is a critical step in designing an effective antisense inhibitor. Here, we report measurements of the thermodynamics and kinetics of hybridization for a number of oligodeoxynucleotides (ODNs) complementary to the rabbit β-globin (RBG) mRNA using a binding assay that facilitates rapid separation of bound from free species in solution. A wide range of equilibrium dissociation constants were observed, and association rate constants within the measurable range correlated strongly with binding affinity. In addition, a significant correlation was observed of measured binding affinities with binding affinity values predicted using a thermodynamic model involving DNA and RNA unfolding, ODN hybridization, and RNA restructuring to a final free energy minimum. In contrast to the behavior observed for hybridization of short strands, the association rate constant increased with temperature, suggesting that the kinetics of association are related to disrupting the native structure of the target RNA. The rate of cleavage of the RBG mRNA in the presence of ribonuclease H and ODNs of varying association kinetics displayed apparent first-order kinetics, with the rate constant exhibiting binding-limited behavior at low association rates and reaction-limited behavior at higher rates. Implications for the rational design of effective antisense reagents are discussed.     

Taking control of gene expression with light-activated oligonucleotides

The recent development of caged oligonucletides that are efficiently activated by ultraviolet (UV) light creates opportunities for regulating gene expression with very high spatial and temporal resolution. By selectively modulating gene activity, these photochemical tools will facilitate efforts to elucidate gene function and may eventually serve therapeutic aims. We demonstrate how the incorporation of a photocleavable blocking group within a DNA duplex can transiently arrest DNA polymerase activity. Indeed, caged oligonucleotides make it possible to control many different protein-oligonucleotide interactions. In related experiments, hybridization of a reverse complementary (antisense) oligodeoxynucleotide to target mRNA can inhibit translation by recruiting endogenous RNases or sterically blocking the ribosome. Our laboratory recently synthesized caged antisense oligonucleotides composed of phosphorothioated DNA or peptide nucleic acid (PNA). The antisense oligonucleotide, which was attached to a complementary blocking oligonucleotide strand by a photocleavable linker, was blocked from binding target mRNA. This provided a useful method for photomodulating hybridization of the antisense strand to target mRNA. Caged DNA and PNA oligonucleotides have proven effective at photoregulating gene expression in cells and zebrafish embryos.     

New oligodeoxynucleotide derivatives containing <Emphasis Type="Italic">N</Emphasis>-(methanesulfonyl)-phosphoramidate (mesyl phosphoramidate) internucleotide group

Novel oligonucleotide derivatives containing N-(methanesulfonyl)-phosphoramidate (mesyl phosphoramidate) group have been described. Solid-phase synthesis of these compounds using an automated DNA synthesizer has been performed for the first time, including the Staudinger reaction between methanesulfonyl azide (mesyl azide) and 3′,5′-dinucleoside 2-cyanoethyl phosphite within an oligonucleotide immobilized on the polymer support, which is a product of phosphoramidite coupling. The mesyl phosphoramidate group is stable to the conditions of oligonucleotide synthesis, in particular, during acidic detritylation and subsequent removal of protecting groups and cleavage of an oligonucleotide from the polymer support by concentrated aqueous ammonia or methylamine at 55°C. It has been shown that the stability of complementary duplexes of oligodeoxynucleotides containing the mesyl phosphoramidate group with a single-stranded DNA is not inferior to the stability of native DNA:DNA duplex. Furthermore, mesyl phosphoramidate oligonucleotides are able to form a complementary duplex with RNA, which is only slightly less stable than the equivalent DNA:RNA duplex. This raises the possibility of their application as potential antisense therapeutic agents.     

In vivo generation of highly abundant sequence-specific oligonucleotides for antisense and triplex gene regulation.

Antisense and triplex oligonucleotides continue to demonstrate potential as mediators of gene-specific repression of protein synthesis. However, inefficient and heterogeneous cellular uptake, intracellular sequestration, and rapid intracellular and extracellular degradation represent obstacles to their eventual clinical utility. Efficient cellular delivery of targeted ribozymes can present similar problems. In this report we describe a system for circumventing these obstacles and producing large quantities of short, sequence-specific RNA oligonucleotides for use in these gene regulation strategies. The oligonucleotides are generated from a vector containing promoter, capping, and termination sequences from the human small nuclear U6 gene, surrounding a synthetic sequence incorporating the oligonucleotide of interest. In vivo, these oligonucleotides are produced constitutively and without cell type specificity in levels up to 5 x 10(6) copies per cell, reach steady-state levels of expression within 9 hours post-transfection, and are still readily detectable 7 days post-transfection. In addition, these oligonucleotides are retained in the nucleus, obtain a 5' gamma-monomethyl phosphate cap, and have an intracellular half-life of approximately one hour. This expression vector provides a novel and efficient method of intracellular delivery of antisense or triplex RNA oligonucleotides (and/or ribozymes) for gene regulation, as well as a cost-effective means of comparing the biological activity arising from a variety of different potential oligonucleotide sequences.     

Effects of RNA secondary structure on cellular antisense activity

The secondary and tertiary structures of a mRNA are known to effect hybridization efficiency and potency of antisense oligonucleotides in vitro. Additional factors including oligonucleotide stability and cellular uptake are also thought to contribute to antisense potency in vivo. Each of these factors can be affected by the sequence of the oligonucleotide. Although mRNA structure is presumed to be a critical determinant of antisense activity in cells, to date little direct experimental evidence has addressed the significance of structure. In order to determine the importance of mRNA structure on antisense activity, oligonucleotide target sites were cloned into a luciferase reporter gene along with adjoining sequence to form known structures. This allowed us to study the effect of target secondary structure on oligonucleotide binding in the cellular environment without changing the sequence of the oligonucleotide. Our results show that structure does play a significant role in determining oligonucleotide efficacy in vivo. We also show that potency of oligonucleotides can be improved by altering chemistry to increase affinity for the mRNA target even in a region that is highly structured.     

Antisense technologies. Improvement through novel chemical modifications.

Antisense agents are valuable tools to inhibit the expression of a target gene in a sequence-specific manner, and may be used for functional genomics, target validation and therapeutic purposes. Three types of anti-mRNA strategies can be distinguished. Firstly, the use of single stranded antisense-oligonucleotides; secondly, the triggering of RNA cleavage through catalytically active oligonucleotides referred to as ribozymes; and thirdly, RNA interference induced by small interfering RNA molecules. Despite the seemingly simple idea to reduce translation by oligonucleotides complementary to an mRNA, several problems have to be overcome for successful application. Accessible sites of the target RNA for oligonucleotide binding have to be identified, antisense agents have to be protected against nucleolytic attack, and their cellular uptake and correct intracellular localization have to be achieved. Major disadvantages of commonly used phosphorothioate DNA oligonucleotides are their low affinity towards target RNA molecules and their toxic side-effects. Some of these problems have been solved in 'second generation' nucleotides with alkyl modifications at the 2' position of the ribose. In recent years valuable progress has been achieved through the development of novel chemically modified nucleotides with improved properties such as enhanced serum stability, higher target affinity and low toxicity. In addition, RNA-cleaving ribozymes and deoxyribozymes, and the use of 21-mer double-stranded RNA molecules for RNA interference applications in mammalian cells offer highly efficient strategies to suppress the expression of a specific gene.     

Antisense oligonucleotide inhibition of hepatitis C virus gene expression in transformed hepatocytes.

Genetic and biochemical studies have provided convincing evidence that the 5' noncoding region (5' NCR) of hepatitis C virus (HCV) is highly conserved among viral isolates worldwide and that translation of HCV is directed by an internal ribosome entry site (IRES) located within the 5' NCR. We have investigated inhibition of HCV gene expression using antisense oligonucleotides complementary to the 5' NCR, translation initiation codon, and core protein coding sequences. Oligonucleotides were evaluated for activity after treatment of a human hepatocyte cell line expressing the HCV 5' NCR, core protein coding sequences, and the majority of the envelope gene (E1). More than 50 oligonucleotides were evaluated for inhibition of HCV RNA and protein expression. Two oligonucleotides, ISIS 6095, targeted to a stem-loop structure within the 5' NCR known to be important for IRES function, and ISIS 6547, targeted to sequences spanning the AUG used for initiation of HCV polyprotein translation, were found to be the most effective at inhibiting HCV gene expression. ISIS 6095 and 6547 caused concentration-dependent reductions in HCV RNA and protein levels, with 50% inhibitory concentrations of 0.1 to 0.2 microM. Reduction of RNA levels, and subsequently protein levels, by these phosphorothioate oligonucleotides was consistent with RNase H cleavage of RNA at the site of oligonucleotide hybridization. Chemically modified HCV antisense phosphodiester oligonucleotides were designed and evaluated for inhibition of core protein expression to identify oligonucleotides and HCV target sequences that do not require RNase H activity to inhibit expression. A uniformly modified 2'-methoxyethoxy phosphodiester antisense oligonucleotide complementary to the initiator AUG reduced HCV core protein levels as effectively as phosphorothioate oligonucleotide ISIS 6095 but without reducing HCV RNA levels. Results of our studies show that HCV gene expression is reduced by antisense oligonucleotides and demonstrate that it is feasible to design antisense oligonucleotide inhibitors of translation that do not require RNase H activation. The data demonstrate that chemically modified antisense oligonucleotides can be used as tools to identify important regulatory sequences and/or structures important for efficient translation of HCV.     

Polymorphisms in the large subunit of human RNA polymerase II as target for allele-specific inhibition

The lack of specificity of cancer treatment causes damage to normal cells as well, which limits the therapeutic range. To circumvent this problem one would need to use an absolute difference between normal cells and cancer cells as therapeutic target. Such a difference exists in the genome of all individuals suffering from a tumor that is characterized by loss of genetic material [loss of heterozygosity (LOH)]. Due to LOH, the tumor is hemizygous for a number of genes, whereas the normal cells of the individual are heterozygous for these genes. Theoretically, polymorphic sites in these genes can be utilized to selectively target the cancer cells with an antisense oligonucleotide, provided that it can discriminate the alleles and inhibit gene expression. Furthermore, the targeted gene should be essential for cell survival, and 50% gene expression sufficient for the cell to survive. This will allow selective killing of cancer cells without concomitant toxicity to normal cells. As an initial step in the experimental test of this putative selective cancer cell therapy, we have developed a set of antisense phosphorothioate oligonucleotides which can discriminate the two alleles of a polymorphic site in the gene encoding the large subunit of RNA polymerase II. Our data show that the exact position of the antisense oligonucleotide on the mRNA is of essential importance for the oligonucleotide to be an effective inhibitor of gene expression. Shifting the oligonucleotide position only a few bases along the mRNA sequence will completely abolish the inhibitory activity of the antisense oligonucleotide. Reducing the length of the oligonucleotides to 16 bases increases the allele specificity. This study shows that it is possible to design oligonucleotides that selectively target the matched allele, whereas the expression level of the mismatched allele, that differs by one nucleotide, is only slightly affected.     

绿色荧光蛋白基因mRNA反义寡核苷酸的筛选和应用

基因mRNA的靶点筛选是设计反义寡核苷酸的关键.建立了PARASS(polyAanchoredRNAaccessiblesitesscreening)方法,即通过在mRNA末端引入polyA,与生物素标记的polyT退火结合,将其同链亲和素磁珠混合,使mRNA通过3’末端得到固定,保持mRNA的自然伸展和折叠,与寡核苷酸文库杂交筛选mRNA的结合靶点.PARASS筛选获得了绿色荧光蛋白(GFP)mRNA的3个反义寡核苷酸结合靶点,据其设计了多条反义寡核苷酸,与对照组相比,体外RNaseH分析显示3个靶点均为有效,在HeLa细胞内针对靶点的反义寡核苷酸能抑制GFP的表达,得到了Northern印迹结果支持.PARASS对反义寡核苷酸药物设计具有应用价值.     

Potent gene-specific inhibitory properties of mixed-backbone antisense oligonucleotides comprised of 2'-deoxy-2'-fluoro-D-arabinose and 2'-deoxyribose nucleotides

Phosphorothioate deoxyribonucleotides (PS-DNA) are among the most widely used antisense inhibitors. PS-DNA exhibits desirable properties such as enhanced nuclease resistance, improved bioavailability, and the ability to induce RNase H mediated degradation of target RNA. Unfortunately, PS-DNA possesses a relatively low binding affinity for target RNA that impacts on its potency in antisense applications. We recently showed that phosphodiester-linked oligonucleotides comprised of 2'-deoxy-2'-fluoro-D-arabinonucleic acid (FANA) exhibit both high binding affinity for target RNA and the ability to elicit RNase H degradation of target RNA [Damha et al. (1998) J. Am. Chem. Soc. 120, 12976]. In the present study, we evaluated the antisense activity of phosphorothioate-linked FANA oligonucleotides (PS-FANA). Oligonucleotides comprised entirely of PS-FANA were somewhat less efficient in directing RNase H cleavage of target RNA as compared to their phosphorothioate-linked DNA counterparts, and showed only weak antisense inhibition of cellular target expression. However, mixed-backbone oligomers comprised of PS-FANA flanking a central core of PS-DNA were found to possess potent antisense activity, inhibiting specific cellular gene expression with EC(50) values of less than 5 nM. This inhibition was a true antisense effect, as indicated by the dose-dependent decrease in both target protein and target mRNA. Furthermore, the appearance of mRNA fragments was consistent with RNase H mediated cleavage of the mRNA target. We also compared a series of PS-[FANA-DNA-FANA] mixed-backbone oligomers of varying PS-DNA core sizes with the corresponding 2'-O-methyl oligonucleotide chimeras, i.e., PS-[2'meRNA-DNA-2'meRNA]. Both types of oligomers showed very similar binding affinities toward target RNA. However, the antisense potency of the 2'-O-methyl chimeric compounds was dramatically attenuated with decreasing DNA core size, whereas that of the 2'-fluoroarabino compounds was essentially unaffected. Indeed, a PS-FANA oligomer containing a single deoxyribonucleotide residue core retained significant antisense activity. These findings correlated exactly with the ability of the various chimeric antisense molecules to elicit RNase H degradation of the target RNA in vitro, and suggest that this mode of inhibition is likely the most important determinant for potent antisense activity.     

Contributions of individual nucleotides to tertiary binding of substrate by a Pneumocystis carinii group I intron

Pneumocystis carinii is a mammalian pathogen that infects and kills immunocompromised hosts such as cancer and AIDS patients. The LSU rRNA precursor of P. carinii contains a conserved group I intron that is an attractive drug target because humans do not contain group I introns. The oligonucleotide r(AUGACU), whose sequence mimics the 3'-end of the 5'-exon, binds to a ribozyme derived from the intron with a K(d) of 5.2 nM, which is 61000-fold tighter than expected from base-pairing alone [Testa, S. M., Haidaris, G. C., Gigliotti, F., and Turner, D. H. (1997) Biochemistry 36, 9379-9385]. Thus, oligonucleotide binding is enhanced by tertiary interactions. To localize interactions that give rise to this tertiary stability, binding to the ribozyme has been measured as a function of oligonucleotide length and sequence. The results indicate that 4.3 kcal/mol of tertiary stability is due to a G.U pair that forms at the intron's splice junction. Eliminating nucleotides at the 5'-end of r(AUGACU) does not affect intron binding more than expected from differences in base-pairing until r((___)ACU), which binds much more tightly than expected. Adding a C at the 5'- or 3'-end that can potentially form a C-G pair with the target has little effect on binding affinity. Truncated oligonucleotides were tested for their ability to inhibit intron self-splicing via a suicide inhibition mechanism. The tetramer, r((__)GACU), retains similar binding affinity and reactivity as the hexamer, r(AUGACU). Thus oligonucleotides as short as tetramers might serve as therapeutics that can use a suicide inhibition mechanism to inhibit self-splicing. Results with a phosphoramidate tetramer and thiophosphoramidate hexamer indicate that oligonucleotides with backbones stable to nuclease digestion retain favorable binding and reactivity properties.     

Recognition of duplex DNA by RNA polynucleotides.

We are interested in creating artificial gene repressors based on duplex DNA recognition by nucleic acids. Homopyrimidine RNA oligonucleotides bind to duplex DNA at homopurine/homopyrimidine sequences under slightly acidic conditions. Recognition is sequence-specific, involving rU.dA.dT and rC+.dG.dC base triplets. Affinities were determined for folded polymeric RNAs (ca. 100-200 nt) containing 0, 1 or 3 copies of a 21 nt RNA sequence that binds duplex DNA by triple helix formation. When this recognition sequence was inserted into the larger folded RNAs, micromolar concentrations of the resulting RNA ligands bound a duplex DNA target at pH 5. However, these binding affinities were at least 20-fold lower than the affinity of an RNA oligonucleotide containing only the recognition sequence. Enzymatic probing of folded RNAs suggests that reduced affinity arises from unfavorable electrostatic, structural and topological considerations. The affinity of a polymeric RNA with three copies of the recognition sequence was greater than that of a polymeric RNA with a single copy of the sequence. This affinity difference ranged from 2.6- to 13-fold, depending on pH. Binding of duplex DNA by polymeric RNA might be improved by optimizing the RNA structure to efficiently present the recognition sequence.     

New oligodeoxyribonucleotide derivatives bearing internucleotide <Emphasis Type="Italic">N</Emphasis>-tosyl phosphoramidate groups: Synthesis and complementary binding to DNA and RNA

N-Sulfonyl phosphoramidate derivatives of oligodeoxyribonucleotides containing N-tosyl phosphoramidate groups are first reported. The synthesis is based on Staudinger reaction between tosyl azide and 3′,5′-dinucleoside β-cyanoethyl phosphite comprising the immobilized oligonucleotide, which is obtained by the phosphoramidite coupling during the solid-phase oligonucleotide synthesis. The N-tosyl phosphoramidate group was stable under conditions of the oligonucleotide synthesis, in particular, upon acidic detritylation followed by the removal of protective groups and cleavage from the polymer support by the treatment with concentrated aqueous ammonia at 55°C. The stability of DNA and RNA duplexes of the model oligonucleotides containing N-tosyl phosphoramidate groups was only slightly lower than that of native DNA:DNA and DNA:RNA duplexes, respectively.